Precision – long-term stability – reliability, does it really matter ?
Sensors for online oil condition measurement (oOCM) have to operate for a long time and must be able to react early when measurement values begin to deviate from the conventional readings. By using a highly sensitive and long-term stable system, the warning threshold can be set accordingly lower which give the operator more time react.
The costs of maintenance are much lower at an early stage due to better planning and lower downtimes. The benefit of an oOCM system therefore increases overproportionately with the precision of the system.
Why temperature is important
In oil condition monitoring, a specific reference temperature is usually defined for monitoring temperature-dependent variables such as the viscosity at 40°C, for example. If this temperature cannot be exactly held constant at the sensor but can at least be measured, the readings can in principle be converted numerically to the desired reference temperature. However, such an extrapolation is subject to larger inaccuracies due to the usually not exactly known temperature dependence and the dynamic mismatch arising from different time constants of viscosity and temperature sensors.
MicroResonant has therefore developed the VDC measuring cell, a temperature-controlled measuring system in which the fluid temperature can be specified in a wide range and with highest accuracy.
This eliminates the need for conversion to reference temperature, and furthermore allows to determine temperature responses of parameters. These data in turn provide additional information on the oil condition.
Why accurate temperature sensing is important for long-term stability
The viscosity of oils is a strongly temperature-dependent property (typ. 4%/°C). If the temperature sensor readings are subject to a slow temporal drift, the associated apparent change in viscosity could be misinterpreted as ageing of the oil.
Temperature drifts of more than 0.25°C/year are not uncommon. This translates to an apparent drift of viscosity of about 1%/year. Due to harsh environmental conditions, these values are often even significantly exceeded.
In order to address this issue MicroResonant uses high quality temperature sensor elements specifically optimized for long-term stability as well as carefully designed precision electronic circuits.